Stable healing of a tendon graft to the adjacent bone is generally considered to be the single most important factor in any type of tendon or ligament reconstruction. Successful incorporation of the graft is primarily dependent on two factors. First, the graft must be fixed in such a way as to maximize the contact area between the graft and the bone, thereby providing the greatest amount of surface area for graft incorporation. Second, the graft fixation must be stable, minimizing the amount of motion between graft and bone. This will minimize the amount of weak fibrous tissue that forms at the bone-graft interface and maximize the degree to which a more stable bone-soft tissue interface develops at the point of bone-graft contact.
One of the specific areas in which this problem of bone-to-tendon graft healing has received the greatest amount of attention is in the area of cruciate ligament reconstruction. Anterior cruciate ligament (ACL) reconstruction in particular has been an area of intense interest. Graft fixation techniques for ACL reconstruction have become an area of intense debate, research, and product development. Graft fixation during the ACL reconstruction procedure will be used as an example to demonstrate the properties of the new fixation concept described here. Other applications, such as but not limited to other types of ligament reconstruction, are obviously possible as well.
One of the graft fixation techniques that has become increasingly popular is interference screw fixation. Many recent advances have been made in improving the pullout strength of tendon grafts when using interference screw fixation. Better tunnel location, tunnel compaction, tighter graft/tunnel fit, improved graft preparation/suturing techniques, and the use of longer, biodegradable screws have all contributed to nearly doubling the pullout strengths obtained from the initial interference fixation studies.
One of the fundamental problems associated with interference screw fixation has remained unchanged, however. More specifically, the presence of the interference screw on one side of the graft limits the bone/graft contact to only a portion of the graft's circumferential area. Histology studies have suggested that in the long term, the most stable bony ingrowth of the graft into the surrounding bone occurs primarily at the outer rim of the bone tunnel. With interference screw fixation this ingrowth is possible only on the side of the graft that is in direct contact with bone; the other half of the graft contacts only the screw and hence is not available for bony ingrowth.
Thus, in practice, there is effectively no bony ingrowth where the interference screw intervenes between the tendon graft and the host bone.
The use of bioresorbable screws may provide the opportunity for additional bone ingrowth after the bioresorbable screw has been resorbed. However, the timing, extent and type of ingrowth occurring on the screw side of the tendon, after the bioresorbable screw has been resorbed, has yet to be fully determined.
In addition to the foregoing, spinning of the tendon graft during insertion of the interference screw is a well-documented problem that is difficult to control once it has begun. This “tendon spin” can damage the graft and result in impingement and less-than-ideal graft positioning, possibly affecting the clinical results.
As a result of the foregoing, one of the arguments for extra-cortical or non-aperture types of fixation, such as graft suspension systems like the ENDOBUTTON™ system or cross-pinning, is that there is, theoretically, circumferential bone/tendon graft contact, making full circumferential bony ingrowth at least a theoretical possibility. However, such distal types of fixation are often less stiff and provide less stable fixation of the graft in the bone tunnel. This decreased stability and subsequent increased graft-tunnel motion may inhibit the formation of a stable graft-bone interface, interfering with graft incorporation into the adjacent bone and the creation of a functionally stable ligament reconstruction. In addition, this increased graft motion has been associated with widening of the bone tunnel. This tunnel widening, thought to be due to the so-called “windshield wiper” and “bungee-cord” motion of the graft within the bone tunnel, is indicative of an unstable graft-bone construct that is prone to failure.
Improving the biologic potential of graft fixation by increasing the native bone/tendon graft contact area, while still compressing the graft and closing the bone tunnel using interference fixation, would be a desirable goal. Maintaining adequate fixation strength when using any new technique is obviously critical, and improving fixation strength while also improving the biologic properties of the fixation method would obviously represent a significant improvement in graft fixation.
This application describes a new method and apparatus to achieve these goals.